A light-emitting diode (LED) is principally formed by a semiconductor material of multiple stacked epitaxial layers. For example, a blue-light LED is mainly consisted of gallium nitride-based (GaN-based) epitaxial thin films.
Referring to FIG. 1, a conventional vertical LED includes a sandwich structure formed by an N-type semiconductor layer 1, a light-emitting layer 2 and a P-type semiconductor layer 3. Below the P-type semiconductor layer 3, a mirror layer 4, a buffer layer 5, a binding layer 6, a silicon substrate 7 and a P-type electrode 8 are disposed in sequence. A surface of the N-type semiconductor layer 1 is processed by a roughening treatment for increasing light extraction efficiency. An N-type electrode 9 is further provided. By applying a voltage to the N-type electrode 9 and the P-type electrode 8, the N-type semiconductor layer 1 is enabled to provide electrons and the P-type semiconductor layer 3 is enabled to provide holes. Light is produced by the electrons and holes combining at the light-emitting layer 2.
In a conventional solution for increasing light extraction efficiency of an LED, light emitted by the light-emitting layer 2 is reflected by the mirror layer 4. Thus, the mirror layer 4 is made of a high-reflectivity material such as a silver/titanium tungsten/platinum alloy coating, a silver/titanium/platinum alloy coating, a silver/titanium tungsten/nickel alloy coating and a silver/nickel alloy coating. Through the mirror layer 4 having the properties of high reflectivity and high thermal stability due to the above materials, light can be reflected at a maximum angle to increase light extraction efficiency and provide stable electrical characteristic.
After forming the mirror layer 4 below the P-type semiconductor 3 of the LED, the buffer layer 5 and the binding layer 6, the silicon substrate 7 and the P-type electrode 8 are yet to be formed, inferring that several semiconductor processes need to be performed in order to achieve a complete LED structure. However, the silver in the mirror layer 4 is prone to oxidation in the subsequent processes, and it may reduce the reflectivity of the mirror layer 4 to further decrease light extraction efficiency of the LED. Further, high temperature produced in high-current operations may also lead to metal deterioration, which similarly reduces the reflectivity of the mirror layer 4 to decrease light extraction efficiency of the LED.